Building little audio gizmos is fun. I recommend using replacement smartphone speakers (eg for the iPhone SE 2020) if size is an issue, because a bare chassis without an enclosure sounds like crap due to acoustic short-circuit.
Another option is LCD TV speakers, those already have an enclosure but are a bit larger.
This looks like the speaker that goes into your ear, but I meant the one at the bottom of your phone that activates when you switch to "speaker phone". They usually have the keyword "buzzer" in the title of sites that sell them to distinguish them from the earpiece.
Some have just two contacts you can easily solder speaker cables to, just avoid the ones with complicated connectors. You don't need to enclose it to get good sound, it already has an enclosure. I often choose plush toys with a simple MP3 SD player (1€ on Aliexpress, eg https://www.aliexpress.com/item/32657798948.html) and some triggering mechanism.
Another option is LCD TV speakers, those already have an enclosure but are a bit larger.